Application of discrete modelling approach to yeast drying
Special Symposium - Innovations in Food Technology (LMC Congress)
Innovations in Food Technology - Poster Session (LMC/Food - P1)
Keywords: Yeast, drying, transport phenomena, porous network
Drying of yeast is of major importance for food industry. Without it, transport and storage of yeast couldn't be easily achieved [1]. Unfortunately, the process has some important drawbacks: it is highly energy consuming and the quality of the product, including viability, can be altered. Study and modelling of yeast drying is then useful to limit energy loss and to minimize product alteration. Classical models of yeast drying are based on desorption isotherms: experimental curves of equilibrium solid moisture content as a function of air moisture content. Such models have several limitations. Firstly, the production of experimental desorption isotherms is a difficult and expensive process. Moreover, the isotherm depends on solid geometry, so if yeast grains are modified, in theory the isotherm measurements have to be repeated. Secondly, models are very sensitive to isotherms. So, a precise model would require a very precise isotherm, which cannot always be obtained experimentally. Thirdly, although isotherms are based on equilibrium, they are often used to model the kinetic of the drying; some kinetics effect cannot be highlighted by such a method.
Most alternative models developed to integrate the effect of solid structure on drying are based on a continuum approach, where the porous system is modelled by a fictive continuous medium [2]. In this paper, a discrete modelling of yeast grain is used. In the discrete approach, the medium is modelled directly at the pore scale by considering a network of pores linked together by throats. Simplified local transport equations for each throat and pore are then solved. Such models naturally take into account phenomena that classical models can’t easily handled as the fractal form of the drying front. It represents a powerful complementary tool to continuous approach [3]. The model used in our simulations includes most features of existing fundamentals pore network models for evaporation: diffusive vapour transport, capillary and viscous effect in the liquid phase and film flow in the partially saturated area [4]. Network parameters of the models are deduced from the analysis of the solid structure observed with an environmental scanning electron microscope.
The model is validated by comparison with experiments of drying of a small quantity of yeast in a thermogravimetry analyser. The practical case of fluidized bed drying is then studied. The model couples the pore network model for yeast grains to classical transport equations for the whole reactor. The results are compared to experimental results on a laboratory pilot plant and to a simple receding front continuous model. This comparison allows highlighting strengths, weaknesses and complementarities of both approaches.
Bibliography
[1] D. Bayrock and W.M Ingledew. Fluidized bed drying of baker’s yeast: moisture levels, drying rates, and viability changes during drying. Food research International, 30(6):407–415, 1997.
[2] S. Whitaker. A theory of drying in porous media. Advances in heat transfer, 13 :119–203, 1977.
[3] M. Prat. Discrete models of liquid-vapour phase change phenomena in porous media. Revue générale de Thermique, 37 :954–961, 1998.
[4] A.G. Yiotis, A.G. Boudouvis, A.K. Stubos, I.N. Tsimpanogiannis, and Y.C. Yortsos. Effect of liquid films on the drying of porous media. AIChE Journal, 50(11) :2721–2737, nov 2004.
See the full pdf manuscript of the abstract.
Presented Wednesday 19, 13:30 to 15:00, in session Innovations in Food Technology - Poster Session (LMC/Food - P1).
